The invention relates to a method for monitoring the load on rotor blades of wind energy installations by means of acceleration measurements on at least one rotor blade during the operation of the wind energy installation and to the determination of evaluation variables for assessing the load on the rotor blade from the acceleration signals picked up.
Rotor blades are among the most highly loaded components of a wind energy installation. They should withstand the enormous centrifugal forces, wind streams and gusts, turbulences, solar irradiation, the most varied temperatures and ice aggregation over a number of years in continuous operation in order to provide for economic operation of the wind energy installation.
Rotor blades, therefore, are also among the components having the highest failure rates. Repairs and the exchange of rotor blades which is still most often carried out at present, are very cost-intensive and cause long periods of lack of income. For this reason, the early detection of damage in the various components of the rotor blade, particularly in the aerodynamic shell of the rotor blade and in its supporting components in the interior of the rotor blade is necessary.
Indicating the damage event provides important information for damage prevention and preventative maintenance. However, it is more productive to detect and indicate the possible causes of damage events. This includes especially dynamic overloads which arise due to strong changes in the wind or turbulences and strong winds and lead to the rotor blades being loaded beyond the design load and can be damaged, as a result. The dynamic loads are alternating loads with high rates of change which therefore are a main cause of rotor blade damage.
Dynamic overloads can also be produced if the rotor blades are not aerodynamically equal or matched to one another, e.g. the pitch control is not appropriate for one blade.
Such conditions must therefore be absolutely avoided. As a rule, the pitch control of the rotor blades, or, in so-called stall-controlled installations in which the rotor blades have a fixed angle setting carried out during their erection, the basic setting of the rotor blades, are responsible for correcting the said or comparable events in such a manner that the overloads do not occur.
It is therefore important to determine the dynamic overloads in order to detect and remedy existing control errors or, respectively, to optimize the pitch control from the findings relating to the dynamic overloads. In the same way, the basic adjustment of stall-controlled wind energy installations can be checked and adapted.
Methods and arrangements have hitherto become known in which strains at particular points in the rotor blades are measured by means of strain gauges and optical fiber systems having the same effect in order to detect and evaluate from these load states for an entire rotor blade in each case. The load states measured in this approach are basically not alternating loads but only quasi-static loads having low rates of change. However, strain gauges have problems in continuous use and are difficult to exchange with constant positioning. On the other hand, measurements of strains with optical fibers are expensive and irreparable when they are destroyed due to excessive strains.
The measurement of accelerations in the rotor blade by means of acceleration sensors is more advantageous as described, for example, in the patent document EP 1 075 600. In principle, the acceleration value already provides load-related information and, in addition, acceleration sensors are comparatively inexpensive and long-lasting and can be exchanged without problems in the case of damage.
For a preventative repair, the type of damage of rotor blades is determined more precisely in DE 100 65 314 A1. For this purpose, non-critical damages to the individual rotor blades are also determined from the determination of the resonant and natural frequencies which are generated in the rotor blades by means of vibration excitation and their comparison with provided reference spectra, characterizing defined damage states. In this context, it is found to be a disadvantage, however, that the damaging events are considered as single events but the fatigue of the material of the rotor blade due to frequently occurring, actually non-critical events is only considered if it has led to damage. The invention is therefore based on the objective of specifying a method by means of which dynamic, also non-critical load states can be detected in the differentiated manner and combined in a suitable form for evaluation in order to influence, if possible prior to the onset of damage to the rotor blade, the mode of operation of the wind energy installation, in particular the design of the pitch control or angle setting of stall-controlled rotor blades, on the basis both of all the rotor blades together and of a single rotor blade.
This object is achieved by means of a method which has the features of claim 1.